diff --git a/server.py b/server.py
index 39ba81c3a1..95a28977c1 100644
--- a/server.py
+++ b/server.py
@@ -184,6 +184,13 @@ def __init__(self, *args, **kwargs):
         self.accept_loop_requests_total = 0
         self.accept_loop_last_request_at = 0.0
 
+    def server_bind(self):
+        if sys.platform == 'win32':
+            self.allow_reuse_address = False
+            SO_EXCLUSIVEADDRUSE = getattr(socket, 'SO_EXCLUSIVEADDRUSE', -5)
+            self.socket.setsockopt(socket.SOL_SOCKET, SO_EXCLUSIVEADDRUSE, 1)
+        super().server_bind()
+
     def _handle_request_noblock(self):
         """Record accept-loop progress before dispatching a request handler.
 
@@ -477,6 +484,25 @@ def _log_shutdown_audit(reason: str = "serve_forever_exit") -> None:
     )
 
 
+def _abort_if_already_serving(host: str, port: int) -> None:
+    """Refuse to start if a live HTTP server is already responding on this port."""
+    probe_host = '127.0.0.1' if host in ('0.0.0.0', '', '::') else host
+    try:
+        with socket.create_connection((probe_host, port), timeout=2) as s:
+            s.sendall(b'GET /health HTTP/1.0\r\nHost: localhost\r\n\r\n')
+            s.settimeout(2)
+            data = s.recv(512)
+            if data:
+                print(
+                    f'[!!] FATAL: Another server is already responding on'
+                    f' {probe_host}:{port}. Stop the existing instance first.',
+                    flush=True,
+                )
+                sys.exit(1)
+    except (ConnectionRefusedError, ConnectionResetError, OSError, socket.timeout):
+        pass
+
+
 def main() -> None:
     from api.config import print_startup_config, verify_hermes_imports, _HERMES_FOUND
 
@@ -572,6 +598,7 @@ def main() -> None:
     except Exception as e:
         print(f'[!!] WARNING: Plugin loading failed: {e}', flush=True)
 
+    _abort_if_already_serving(HOST, PORT)
     httpd = QuietHTTPServer((HOST, PORT), Handler)
 
     # ── TLS/HTTPS setup (optional) ─────────────────────────────────────────
@@ -597,6 +624,7 @@ def main() -> None:
     try:
         httpd.serve_forever()
     finally:
+        httpd.server_close()
         _log_shutdown_audit()
         # Stop the gateway watcher on shutdown
         try:
diff --git a/tests/test_server_port_exclusivity.py b/tests/test_server_port_exclusivity.py
new file mode 100644
index 0000000000..89eaf4176d
--- /dev/null
+++ b/tests/test_server_port_exclusivity.py
@@ -0,0 +1,90 @@
+"""Duplicate-instance guard: a second server on the same port must be detected
+and refused before bind, not silently shared (#3289)."""
+
+from __future__ import annotations
+
+import socket
+import sys
+import threading
+from http.server import BaseHTTPRequestHandler, HTTPServer
+
+import pytest
+
+from tests._pytest_port import TEST_PORT
+
+
+# ── SO_EXCLUSIVEADDRUSE on Windows ──────────────────────────────────────────
+
+@pytest.mark.skipif(sys.platform != 'win32', reason='Windows-only socket option')
+def test_exclusive_addr_use_set_on_windows():
+    from server import QuietHTTPServer
+    port = TEST_PORT + 901
+    httpd = QuietHTTPServer(('127.0.0.1', port), BaseHTTPRequestHandler)
+    try:
+        val = httpd.socket.getsockopt(
+            socket.SOL_SOCKET,
+            getattr(socket, 'SO_EXCLUSIVEADDRUSE', -5),
+        )
+        assert val != 0, 'SO_EXCLUSIVEADDRUSE should be set on Windows'
+    finally:
+        httpd.server_close()
+
+
+# ── Live-listener probe ─────────────────────────────────────────────────────
+
+def test_probe_detects_live_server():
+    """_abort_if_already_serving must call sys.exit when a live server responds."""
+    from server import _abort_if_already_serving
+
+    port = TEST_PORT + 902
+
+    class Handler(BaseHTTPRequestHandler):
+        def do_GET(self):  # noqa: N802
+            self.send_response(200)
+            self.end_headers()
+            self.wfile.write(b'ok')
+        def log_message(self, *a):
+            pass
+
+    httpd = HTTPServer(('127.0.0.1', port), Handler)
+    t = threading.Thread(target=httpd.serve_forever, daemon=True)
+    t.start()
+    try:
+        with pytest.raises(SystemExit):
+            _abort_if_already_serving('127.0.0.1', port)
+    finally:
+        httpd.shutdown()
+        httpd.server_close()
+
+
+def test_probe_allows_startup_when_nothing_listening():
+    """_abort_if_already_serving must return normally on a free port."""
+    from server import _abort_if_already_serving
+
+    port = TEST_PORT + 903
+    _abort_if_already_serving('127.0.0.1', port)
+
+
+def test_probe_allows_startup_on_unresponsive_socket():
+    """A socket that accepts but never responds (e.g. dying instance still in
+    kernel backlog) should not block startup."""
+    from server import _abort_if_already_serving
+
+    port = TEST_PORT + 904
+    srv = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
+    srv.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
+    srv.bind(('127.0.0.1', port))
+    srv.listen(1)
+    try:
+        _abort_if_already_serving('127.0.0.1', port)
+    finally:
+        srv.close()
+
+
+def test_probe_normalizes_wildcard_host():
+    """0.0.0.0 and :: should probe 127.0.0.1, not the literal wildcard."""
+    from server import _abort_if_already_serving
+
+    port = TEST_PORT + 905
+    _abort_if_already_serving('0.0.0.0', port)
+    _abort_if_already_serving('::', port)